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Chapter 3. AntigensTerminology:

Antigen: Substances that can be recognized by the surface

antibody (B cells) or by the TCR (T cells) when associated with MHC molecules

Immunogenicity VS Antigenicity:

Immunogenicity – ability to induce an antibody and/or cell-mediated immune response

Antigenicity – ability to combine with the final products of the response (antibodies and/or T cell receptor)

NOTE: Most immunogenic molecules are also antigenic

Figure 5.1

Hapten - a small molecule that is antigenic but not

(by itself) immunogenic.

Antibodies can be made to haptens only after the

hapten is covalently conjugated to a large protein

“carrier”.

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2

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Factors that influence immunogenicity:

- Foreign-ness – non-self (far apart evolutionary)

- Type of molecule (chemical nature) - protein >

polysaccharide > lipid > nucleic acid

- Size - larger molecules tend to be more immunogenic

-Composition - heterogeneity increases immunogenicity.- 4ry > 3ry > 2ry > 1ry structure

-Degradability - protein antigens must be degraded (phagocytosis) in order to be presented to helper T cells.

- Physical Form - Denatured > Native

Additional factors that influence the immune response:

- Genetics of the recipient (genotype - MHC)

- Dosage of the antigen (optimal dose - tolerance)

- Number of doses of the antigen (boosters)

- Route of administration of the antigen

- intravenous (spleen)- subcutaneous (lymph nodes)- intraperitoneal (lymph nodes)- oral (mucosal)

- inhaled (mucosal)

- Use of adjuvant

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Adjuvant: a substance that, when mixed with an antigen

and injected with it, serves to enhance the immune response to the antigen.

Possible mechanisms of action of adjuvants:

- Prolong the persistence of the antigen, thus giving theimmune system more time to respond

- Increase the “size” of the antigen by causing aggregation,

- Stimulate lymphocyte proliferation and/or activation

- Stimulate a local inflammatory response, thus recruiting cells to the site of the antigen (GRANULOMA)

- Enhance co-stimulatory signals

Commonly used adjuvants: (Table 3.3)

Alum - aluminum potassium sulfate - precipitates the antigen, resulting in increased persistence of the antigen and induces mild granuloma.

Incomplete Freund’s adjuvant - mineral oil-based - increasespersistence of the antigen, mild granuloma, and induces co-stimulatory signals

Complete Freund’s Adjuvant - mineral oil-based adjuvantcontaining dead bacteria - increases persistence of the

antigen, stimulates a chronic inflammatory response (granuloma), and co-stimulatory signals

Bacterial Lipopolysaccharides - stimulate nonspecific lymphocyte activation and proliferation, and co-stimulatory signals.

Epitope or Antigenic Determinant - the region

of an antigen that binds to a T cell receptor or a B

cell receptor (antibody).

- Since an epitope is the part of the antigen that

binds to the B cell or T cell antigen receptor, it is

the part that determines the antigenicity of the

antigen - thus the term “antigenic determinant”.

-T and B cells recognize different epitopes on an

antigen

- Each different protein and glycoprotein of a virus (or bacterium

or foreign cell) constitutes a different antigen

- Each different antigen contains a number of different epitopes

INCREASED COMPLEXITY

Properties of B cell epitopes (Table 3-4)

- Usually dependent on the native, tertiary conformationof the antigen (PROTEIN FOLDING)

- Must be accessible - tend to be on the “surface” of the antigen (hydrophilic)

- May be made of sequential or non-sequential amino acid sequences (epitopes made up of non-sequential amino

acid sequences are called “conformational epitopes”).

- Binds to soluble antigen, No MHC molecule requirement

- Large antigens contain multiple, overlapping B cell

epitopes.

PROTEIN

FOLDING

Denaturation!!!!

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Properties of B cell epitopes (Table 3-4)

- Usually dependent on the native, tertiary conformation of the antigen

- Must be accessible - tend to be on the “surface” of the antigen (hydrophilic)

- May be made of sequential or non-sequential amino acid sequences (epitopes made up of non-sequential amino

acid sequences are called “conformational epitopes”).

- Binds to soluble antigen, No MHC molecule requirement

- Large antigens contain multiple, overlapping B cell

epitopes.

HYDROPHILIC

Properties of B cell epitopes (Table 3-4)

- Usually dependent on the native, tertiary conformation of the antigen

- Must be accessible - tend to be on the “surface” of the antigen (hydrophilic)

- May be made of sequential or non-sequential amino acid sequences (epitopes made up of non-sequentialamino acid sequences are called “conformationalepitopes”).

- Binds to soluble antigen, No MHC molecule requirement

- Large antigens contain multiple, overlapping B cell epitopes.

Anti-lysozyme (+)

Anti-lysozyme (-)

Anti-lysozyme (+)

1. Antibody binding may be lost after a protein is denatured!!2. ???

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A

B

Properties of B cell epitopes (Table 3-4)

- Usually dependent on the native, tertiary conformation of the antigen

- Must be accessible - tend to be on the “surface” of the antigen (hydrophilic)

- May be made of sequential or non-sequential amino acid sequences (epitopes made up of non-sequential amino acid

sequences are called “conformational epitopes”).

- Binds to soluble antigen, No MHC molecule requirement

- Large antigens contain multiple, overlapping B cell epitopes.

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animation and pictures from http://www.cat.cc.md.us/courses/bio141/lecguide/unit3/epsig.html

"B-lymphocytes have sIg molecules on their surface that recognize

epitopes directly on antigens. Different B-lymphocytes are

programmed to produce different molecules of sIg, each specific

for a unique epitope."

Properties of B cell epitopes (Table 3-4)

- Usually dependent on the native, tertiary conformation of the antigen

- Must be accessible - tend to be on the “surface” of the antigen (hydrophilic)

- May be made of sequential or non-sequential amino acid sequences (epitopes made up of non-sequential amino acid

sequences are called “conformational epitopes”).

- Binds to soluble antigen, No MHC molecule

requirement

- Large antigens contain multiple, overlapping B cell epitopes.

Large antigens contain multiple,

overlapping B cell epitopes.

Linear or Sequential Antigen

1 2010 30 110

Epitope 1Epitope 2Epitope 3

Amino acids 1-12Amino acids 8-20

Amino acids 19-33

Would this cause cross-reactivity?

Properties of T cell epitopes (Table 3-4)

- Involves a tertiary complex: T cell receptor, antigen, and MHC molecule

- Must be accessible - tend to be on the “surface” of the antigen (hydrophilic)

- May be made of sequential or non-sequential amino acid sequences (epitopes made up of non-sequentialamino acid sequences are called “conformational

epitopes”).

- Binds to soluble antigen, No MHC molecule requirement

- Large antigens contain multiple, overlapping B cell

epitopes.

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2

1

4

Properties of T cell epitopes (Table 3-4)

- Involves a tertiary complex: T cell receptor, antigen, and MHC molecule

- Internal linear peptides (hydrophobic) produced by processing and bound to MHC molecules

- Binds to soluble antigen, No MHC molecule requirement

- Large antigens contain multiple, overlapping B cell

epitopes.

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Properties of T cell epitopes (Table 3-4)

- Involves a tertiary complex: T cell receptor, antigen, and MHC molecule

- Internal linear peptides (hydrophobic) produced by

processing and bound to MHC molecules

- Does not bind to soluble antigen, APC processing

- Recognize mostly proteins but some lipids and glycolipids

can be presented on MHC-like molecules

Must be processed & presented with MHC in APC!!!!

Properties of T cell epitopes (Table 3-4)

- Involves a tertiary complex: T cell receptor, antigen, and MHC molecule

- Internal linear peptides (hydrophobic) produced by

processing and bound to MHC molecules

- Does not bind to soluble antigen, APC processing

- Recognize mostly proteins but some lipids and glycolipids can be presented on MHC-like molecules (remember CD1 molecules!)

Good exam question!!!Pattern-Recognition Receptors (PRR)

- Receptors of the innate immune system

- Recognize unique antigens (motifs) in micro-

organisms (Danger Signals!!!)

- These antigens are absent in the host (non-

self)

- Several Patter-Recognition Receptors (PRRs)

identified

- BIO401: Toll-like receptors (TLRs)

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Macrophage

recruitment &

activation;

Secretion of

inflammatory

cytokines

TLR-2 (Toll-like

receptor 2)

Lipoproteins of

Gram + bacteria

Yeast cell wall

components

PhagocytosisScavenger receptorsPolyanions

Opsonization;

Complement

activation

Mannose-binding

protein

Mannose-

containing

carbohydrates

Opsonization;

Complement

activation

ComplementMicrobial cell wall

components

Biological

Consequence of

InteractionPRRPAMP

Production of

interferon (antiviral)

TLR-3Double stranded

RNA

Macrophage

recruitment &

activation; Secretion

of inflammatory

cytokines

TLR-4LPS

(lipopolysaccharide

of Gram – bacteria

Biological

Consequence of

InteractionPRRPAMP

Macrophage

recruitment &

activation; Secretion

of inflammatory

cytokines

TLR-9CpG motifs in

prokaryotes

Macrophage

recruitment &

activation; Secretion

of inflammatory

cytokines

TLR-5Flagellin (bacterial

flagella)

Biological

Consequence of

InteractionPRRPAMP

Secretion of Inflammatory Cytokines activates T cells

and NK cells!!!!!

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Activation through TLRs ���� secretion of pro-inflammatory

cytokines and cell recruitment